The primary system of a nuclear energy plant includes steam generators, a reactor vessel and core, pipes interconnecting the same and pumps to move water therethrough. Over years of use, the steam generator tubes may undergo degradation necessitating selective plugging of those tubes, which decreases the thermal and hydraulic performance of the steam generator. As the number of plugged tubes increases, the potential for unit derating and for additional licensing restrictions increases. Therefore, a decision must be made as to whether it is more economical to replace the steam generator or to continue with reduced efficiencies. The replacement of a steam generator theoretically restores the plant to its initial condition to allow performance at or near the intended specifications.
Several methods have been used to replace steam generators in nuclear energy plants. The first method included removing sections of inlet and outlet pipes in the primary system to allow the steam generator to be removed. The steam generator was then either replaced in whole or in part, and the new steam generator lowered back into place. The pipe sections were then reinstalled to complete the system. This method required at least two cuts to be made on each inlet and outlet pipe in a radioactive environment for steam generator removal. This method for reinstallation also required pipe realignment and reworking to be made in a radioactive environment followed by at least two welds on each pipe in that environment. This process required the entire plant to be shut down for a relatively extended period of time and resulted in the workers performing most of the replacement work in a radioactive environment.
Another replacement method, developed by the assignee of the present invention, utilized a template to assist in aligning the new vessel with the old pipes. In this method, the new steam generator had its support base and its inlet and outlet nozzles finish machined. The template was constructed on the new steam generator with rings centered on the inlet and outlet nozzle weld center lines and a base structure centered about the support base. A framework was then built to interconnect the rings and base structure to rigidly hold the same in their respective, centered locations. The relative positions of the nozzle weld center lines to the rings therearound was then measured and recorded.
The template was then removed from the new steam generator for placement on the cut ends of the existing primary system coolant pipes after the old steam generator had been disconnected and removed from the system. Measurements were then made of the cut end pipes of the primary system relative to the respective rings of the templates positioned therearound. These measurements were then compared to the measurements on the new steam generator to determine if alignment within weld tolerances existed between the new steam generator nozzles and the pipe ends of the primary system. If not, the cut ends of the primary system pipes were then reworked and/or remachined to achieve the required dimensional alignment to allow the new vessel to be lowered into the containment chamber and connected to the reworked pipe ends. This method was preferable to the earlier method since only one cut was made on each steam generator pipe in the primary system to reduce exposure to radioactivity and to reduce maintenance down time. However, the reworking of the primary system pipes was still being done in a radioactive environment on radioactive pipes, with this work being done while the plant was shut down.